1. Field of the Invention
The present invention relates to a heat-sensitive recording sheet, and more particularly to a heat-sensitive recording sheet excellent in dynamic coloring characteristics and stability in storage.
2. Prior Art
Usually, a heat-sensitive recording sheet is made in the following manner. First, colorless or light-colored basic dyes and an organic developer such as a phenol-based substance are respectively ground to finely divided particles. After mixing them, a binder, filler, sensitivity improver, lubricant and other auxiliaries are added to the resultant solution to obtain a liquid for coating. The liquid for coating thus obtained is painted on a support such as a sheet, film or the like to obtain the heat-sensitive recording sheet. The resultant heat-sensitive recording sheet can obtain a colored record by a momentary chemical reaction with application of heat.
The heat-sensitive recording sheet is applied to a wide range of fields such as measuring recorders, terminal printers of computers, facsimile, automatic ticket vending machines, bar code labels, etc. Recently, the quality required for the heat-sensitive recording sheet has become higher with the diversification and high effectiveness of the recording machines. For example, with recording at high speed, highly dense and distinct colored pictures are required to be obtained even by a very low thermal energy. On the other side, a heat-sensitive recording sheet is required which is excellent in preservability such as plasticizer-resisting property, oil resisting property or water resisting property.
The latter requirement comes from the following problems. Namely, printed parts (colored parts) are decolored if sebaceous matters adhere to pictures or the pictures contact with a plasticizer (DOP, DOA, etc.) contained in a wrap film such as a PVC film or the like, salad oil, edible vinegar, water, etc. after the recording of the pictures. Further, if a liquid containing a polar volatile solvent, such as, for example, ethanol, whiskey or hairdressing, is splashed on a recording sheet, the texture portion of the sheet is colored. Both of these disadvantages are significant problems in using, handling and storing the thermal-sensitive recording paper.
Various methods of improving these problems are disclosed in the following public reports. They include a method in which a protective layer of a macromolecular compound such as a water-soluble resin is formed on the surface of the thermal-sensitive recording paper (Japanese Patent Laid-Open No. 57-144793), a method in which a metal salt of alkylxanthic acid is used as a developer for a thermal-sensitive layer on which an overcoat layer is formed (Japanese Patent Laid-Open No. 58-132592), a method in which two specific kinds of phenolic compounds are used as developers for a thermal-sensitive layer on which an overcoat layer is formed (Japanese Patent Laid-Open No. 58-136493), a method in which a specific developer and a specific auxiliary color coupler are added in forming a thermal-sensitive layer on which an overcoat layer is formed (Japanese Patent Laid-Open No. 58-151297), a method in which the thermal-sensitive recording paper is made by layering an undercoat layer principally consisting of a filler and a binding agent, a thermal-sensitive layer and an overcoat layer principally consisting of a water-soluble macromolecular binding agent and a filler in that order over a supporting body and a back-coat layer principally consisting of a water-soluble macromolecular binding agent is formed on the other surface of the supporting body (Japanese Patent Laid-Open No. 58-203092) and a method in which a polyvalent metal salt of a carboxylic acid is added in forming an overcoat layer or an undercoat.
These proposals have the common basic characteristic of physically preventing a plasticizer, salad oil, vinegar or a liquid containing a polar volatile solvent from penetrating into the thermal-sensitive recording layer by coating it with the overcoat layer, the undercoat layer and the back-coat layer. But, microscopic pinholes in these barrier layers would allow the above substance to permeate into the thermal-sensitive recording layer resulting in discoloration of colored picture images or coloration of uncolored areas.
Further, it is practically impossible to produce a film free of pinholes when forming the overcoat layer, the undercoat layer and the heat-sensitive layer on a support sheet on which the heat-sensitive layer is formed.
The inventor considered that those problems may be most perfect solved by providing a heat-sensitive recording layer in which colored images are not discolored and uncolored areas are not colored even when a plasticizer, salad oil, vinegar or a liquid containing a polar volatile solvent permeates through pinholes in the barrier layers into the heat-sensitive layer.